Wipers, certain types of toilet paper, paper napkins and the like are packed in stacks of a fixed height. They are made starting from sheets of absorbent material, for example "tissue" paper, non-woven fabric, etc.
The production starts from a web having a large transversal width, from which sheets are obtained that are folded, stacked and divided into small stacks of a height equal to the final product. Each of these small stacks forms a log of a length equal to said transversal width. The logs, then, are cut off into many short stacks having the final size and packed.
In some cases, the sheets obtained from the web are folded separately from one another and then stacked already folded. In other cases, the sheets are interfolded, i.e., are folded into panels by overlapping at the same time a panel of a previous sheet with a panel of a following sheet. In this way, when pulling a sheet from the stack, at the moment of the use also a panel of the following sheet is pulled out, with consequent advantages for certain types of users. Among the possible interfolding ways the L, Z or W interfolded sheets are known having 2, 3 and 4 panels respectively.
For the production of interfolded stacks machines are known that use one or two webs of paper coming from a reel that are cut into sheets and then supplied offset with respect to one another on folding counter-rotating rollers.
More precisely, cutting rollers that engage with respective blades carry out the cutting step of the webs into sheets. In case of L or W interfolding the webs are cut and then supplied in order to form a sequence of sheets coming preferably from two different directions. Therefore, the sheets coming from both directions are supplied alternately to the folding rollers so that each sheet coming from a first direction overlaps a portion of the sheet coming from the second direction, and vice versa.
The sheets coming from both directions, in order to be overlapped in the above-described way, adhere to the respective folding rollers by means of a sucking step or by means of a mechanical gripping. Therefore, the downstream portion of each sheet leaves its folding roller at the point of contact between the two rollers, then adhering to the other folding roller, to which the upstream portion of the previous sheet adhered.
The method for Z interfolding is similar to what described, with the difference that the overlap between two consecutive sheets occurs just after the cutting step and a sequence of overlapping and offset sheets come to the folding rollers from only one direction.
The interfolded stacking step is accomplished by folding arms that have an oscillating motion about a pivot and that separate in turn from the respective roller the upstream portion of each sheet joined to the overlapped downstream portion of the following sheet. The folding arms are normally arranged in two rows and operate alternatively with the paired portions of sheets, which adhere to the first or to the second folding roller.
The folding rollers have a plurality of circumferential grooves, into which the ends of said folding arms go without blocking their rotation. At the passage of two overlapped portions of two consecutive sheets, that adhere to a roller and cover an end of the folding arms, the folding arms rotate so that their ends go out the grooves and push the two overlapped portions away from the roller, thus folding them onto previously interfolded sheets stacked below.
The folding rollers have a circumference that is normally two times the length of the sheets. Therefore, a sheet adds to the stack of interfolded sheets at each fourth of turn of the folding rollers.
In FIG. 8 an example is shown of folding rollers 57a and 57b according to the prior art having a circumference two times with respect to the length of the sheets. They comprise first sucking spots 58 that are active upstream of the point of contact P between the two rollers and second sucking spots 59 that are active downstream of the point of contact P between the two rollers. In such a point in turn the holes 58 of a roller substantially coincide with the holes 59 of the other roller. The overlap between two sheets 54a and 54b adhering to the two rollers occurs downstream of point P and folding arms 50a and 50b are provided that partially engage with grooves 55 and rotate about pivots 51 at a moment in which the separation from the rollers of partially overlapping sheets 54a and 54b can be carried out. The pivots 51 are distant from folding rollers 57a and 57b.
It is however felt the need of increasing the width of the webs to interfold, thus obtaining longer stacks to cut off. This causes, however, the unavoidable deflexion of the folding rollers, that already are weakened for the presence of the grooves and of the holding means. Deflected rollers cannot work and must be, therefore, stiffened, by increasing their cross section, thus increasing the number of sheets that can be interfolded at each turn. For example, folding rollers with circumference three times the length of the sheets allow a production of one interfolded sheet each sixth of turn, and folding rollers with circumference four times with respect to the length of the sheets allow a production of one interfolded sheet at each eighth of turn.
However, for the folding arms the increase of diameter has the consequence of more fatigue and a larger length. In particular, for causing the ends of the folding arms to go completely into the grooves and to be parallel to the interfolded stack of sheets being formed, and in order to assure a correct folding step, the pivot of rotation of the folding arms must be more distant from the folding rollers, the grooves must be deeper, and the folding arms are thus too long. Therefore, the present shapes and mechanisms of the folding arms are not appropriate for an increase of diameter.